Solid state laser disks have an active thin disk gain medium that is mounted along its back surface to a heat sink. The back surface is highly reflective at the laser and pump wavelengths and the front surface is highly transmissive at the laser and pump wavelengths. An external laser beam enters the disk at its front surface, or “face”, propagates to the back surface, is reflected by the back surface back to the front surface, where it leaves the disk. While propagating in the disk, the laser beam extracts energy from the disk.
The energy lost is restored by the pumped laser beam which also enters the disk at its front surface, propagates to the back surface, is reflected and propagates to the front surface where it leaves the disk. While propagating through the disk the pump beam deposits energy into the disk and stimulates the medium. An example active medium is Yb:YAG (ytterbium-doped yttrium aluminum garnet) which is provided in a circular disk format that may be 10 to 2000 millimeter (mm) in diameter and 200 micron (0.20 mm) thick. A pumping light source is typically provided by a diode laser array which provides a greater than unity gain at the laser wavelength, so as to have the laser disk function as an active mirror. The pumping beam is presented to the face at an angle of incidence that avoids interference with the laser beam.
Laser disk output beam power may be increased by increasing the power delivered by the pumping light source, and/or by combining multiple laser disks in a laser disk resonator. A stable laser disk resonator is used to combine the individual output laser power of multiple disks into a multi kilowatt laser output beam used, for example, in metal working applications. The high power output beams produced by stable laser disk resonators are multimode, and are typically fifty times diffraction limit. While this is suitable for industrial applications, such as laser welding, it is not suitable for applications which require a more precise and tightly focused output laser beam. It is, therefore, desirable to provide a multiple laser disk resonator that produces a high energy, near diffraction limited laser beam that may be used for more precise laser beam applications.